1. Field of the Invention
This invention relates to a wave-shaping circuit for digital signals, and more particularly to a wave-shaping circuit for digital signals which is suitable for the digital signal transmitted through a magnetic recording/reproducing apparatus.
2. Description of the Prior Art
A PCM recording/reproducing apparatus is known wherein a coded audio signal (PCM signal) is recorded with a helical scan type VTR (Video Tape Recorder), and which can be reproduced. In the PCM recording/reproducing apparatus, the frequency of the PCM data (for example, 1.3 to 1.4 MH) is very close to the maximum transmission frequency of the recording/reproducing apparatus so as to transmit a PCM signal of high resolution, or digital signals having large numbers of bits. In that case, the reproduced PCM signal is not rectangular ("0" and "1") but is sinusoidal. Further, time base error components (Jitter) due to fluctuation of the tape running speed and the head drum rotation speed in the recording/reproducing apparatus are included in the reproduced PCM signal. Further, the level of the reproduced PCM signal varies with transmission gain (characteristic of electro-magnetic transducing system), and with time.
Accordingly, it is required that the reproduced signal be sliced or clipped with an accurate threshold level for wave-shaping to form a proper digital signal.
In conventional wave-shaping circuits of one kind, the level of the reproduced signal is fixed with an AGC circuit, and the reproduced signal is sliced or clipped with a fixed threshold level. In conventional wave-shaping circuits of another kind, the pedestal portion of the reproduced signal having a format of video signal is clamped to a fixed level, the peak of the reproduced signal is detected, and the clamped reproduced signal is clipped at a level of 50% of the peak value.
The conventional wave-shaping circuits of one kind have the problem that the AGC voltage detecting circuit and the AGC amplifier have some errors, and so a digital signal of accurate pulse width cannot be obtained. Further, the AGC voltage detecting circuit responds with some delay and some time constant may be present. Accordingly, such circuits have a further problem in that the AGC circuit cannot follow high speed searching mode with very much level variations.
In the conventional wave-shaping circuits of the other kind, the peak detector responds with a delay to the reproduced signal to be detected. Accordingly, the reproduced signal to be clipped should be delayed and a delay line is required which makes such circuit complicated. A pseudo signal such as a ringing signal is included in the reproduced PCM signal having the format of a video signal, and it has some influence on the threshold level. Accurate wave-shaping operation cannot be obtained. Further, the transmission characteristic of the recording medium is non-linear. Accordingly, when the reproduced signal is clipped with a threshold level of 50% of the peak value of the reproduced signal, the pulse widths of the digital signals "1" and "0" are not equal to each other.
Since the clipping level is not proper for the reproduced signal, the error bits are increased. The reproduced analogue signal is distorted and click noise occurs.